1. Field of the Invention
This invention relates to a filled, thermoplastic polymer composition containing a reinforcement promoter, and a method for its production. The term "reinforcement promoter" refers to chemicals which provide both improved tensile strength and ductility when combined with a filled thermoplastic polymer.
2. Description of the Prior Art
A broad range of chemicals have been evaluated as filler treatments or interfacial agents in filled polymers with and without the addition of free radical initiators, such as peroxides. Unfortunately, the literature terminology is usually ambiguous and often erroneous. For example, the terms "coupling agent" or "adhesion promoter", which imply that the additives increase the adhesion or bonding between the filler particle and the surrounding polymer matrix, are often used uncritically. Usually there is no proof of any adhesion effect, and the particular additive may function merely as a filler dispersing aid and, sometimes, as a processing aid by reducing the viscosity of the molten, filled composite. In many cases, the mechanical properties reported for the filled polymers even imply that the additive facilitates release of the matrix polymer from the filler particles, such that the so-called coupling agent actually has a decoupling or debonding effect.
The varied behavior of filler treatment additives in filled polymers may be more clearly envisioned with the help of a composite property chart such as that shown in the FIGURE. On this chart, the abscissa or "x" axis represents the elongation at break and the ordinate or "y" axis represents the maximum tensile strength of a filled polymer. The chart presents the relative strength and ductility of filled polymers for specific polymers containing a specified filler loading, but which vary according to the type of interfacial agent which is added to the composition. The area near "A" represents these properties for a filled composite without any interfacial agent added or where the interfacial agent is ineffective for increasing tensile strength or elongation at break. In general, for filler loadings in a range of about 50 weight percent, the tensile strength and the elongation at break will both be quite low, i.e., the filled composite is both weak and brittle. Certain currently used interfacial agents and filler treatments result in increases in tensile strength with little or no increase in elongation such that although the materials get stronger, they remain brittle. These composites are grouped in the area from "A" to "B" and the interfacial agents producing this effect will here be called coupling agents in the strict sense of the word. Other commonly used additives result in gains in elongation at break with little changes or even decreases in tensile strength such that although such composites can become more ductile, they remain weak, and are often best characterized as "cheesy". These composites are grouped in the area from "A" to "C" and the interfacial agents producing this effect will here be termed decoupling agents. Clearly, the interfacial agents which cause a filled polymer to become both strong and tough, i.e., which causes improvements in both tensile strength and ductility, are by far the commercially most attractive composites. These composites would be grouped in the area from "A" to "D". However, not all interfacial agents can be clearly defined as reinforcing, coupling, decoupling or ineffective since, as can be seen from the FIGURE, no sharp boundaries exist between the designated areas. This is particularly evident for composites which exhibit only modest increases in strength or elongation at break, i.e., which approach area "A" in the FIGURE, or composites made with lower filler loadings.
The dramatic mechanical property improvements attained in vulcanized rubbers by melt compounding with carbon black and certain silica and silicate fillers are well known--such that without the use of these so-called reinforcing fillers the commercial utility of many elastomers, especially the amorphous rubbers, would be severly limited. Encouraged by the filler .[.reinforcemnt.]. .Iadd.reinforcement .Iaddend.response in cured rubbers, many attempts have been made to achieve similar effects in other polymers, especially in thermoplastics. These efforts have to date met with limited success and only for special filler/polymer combinations. In particular, the polyolefins have been notably unresponsive to reinforcement by particulate mineral fillers as would be expected from the unreactive chemical structure of polyolefins.
Early attempts at reinforcing polyethylene with carbon black resulted in stiff, brittle composites of little commercial value. However, it was found that by cross-linking a carbon black/polyethylene blend either by ionizing radiation or by a free radical initiator, such as peroxide, strong and tough thermoset composites could be made. See E. M. Dannenberg et al., Journal of Polymer Science, Volume 31, pages 127-153, 1958. Of course,, cross-linking removes many desirable attributes of the thermoplastic polyolefins such as the facility of using low-cost thermoplastic molding methods, the post-forming ability, such as vacuum and thermoforming, the ability to reprocess scrap and rejects, and more, all of which has limited the commercial usage of this discovery.
A second technically successful approach for reinforcing filled thermoplastic polymers has been developed more recently and is described in U.S. Pat. No. 4,187,210 (Howard, Jr.), issued Feb. 5, 1980. By this technique, an olefin polymerization catalyst is deposited on the filler surface after which the polymer is formed directly on each filler particle resulting in a filled polyolefin composite, which is exceptionally strong and tough. This method has proven useful in preparing filled composites from so-called ultra-high molecular weight (UHMW) polyethylene, i.e., polyethylene having such extreme melt viscosities that conventional melt processing such as injection molding, extrusion, melting, calendering, and the like, is not possible. In this case, incorporation of a filler by melt compounding is not possible and direct polymerization on the filler surface is thus the only feasible alternative. The resulting filled UHMW polyethylene powder may subsequently be formed by powder metallurgical processes such as pressure sintering the powder, e.g., into a billet, followed by forging, skiving, turning, etc. The properties attainable in such composites have been discussed by E. G. Howard et al., in a talk entitled "Ultrahigh Molecular Weight Polyethylene Composites: A New Dimension in Filled Plastics", recorded on pages 36-38 of the preprints of the October 1976 National Technical Conference of the Society of Plastic Engineers. However, for reasons of logistics and cost, it is commercially unattractive to use this technique for polyolefins in the more conventional molecular weight ranges which are capable of standard plastic processing. Hence, this technique has also found limited commercial usage.
A third attempt at reinforcing filled thermoplastics related to both of the above techniques was aimed at coating the filler particles with a layer of cross-linked polymer as described in U.S. Pat. No. 3,471,439 (Bixler et al.), issued Oct. 7, 1969. This was attempted by adsorbing a broad variety of unsaturated organic compounds onto the filler particles and then melt compounding the treated filer with a polymer and a free radical initiator, such as an organic peroxide, a percarbonate or an azo-compound. In contrast to the present invention, the patent discloses that the use of a free radical initiator is preferred and produces better results than in the absence of a free radical initiator. Furthermore, little distinction is made regarding the nature of possible coupling or decoupling effects of the individual filler treatments. This technique was subsequently replaced by an in-situ filler coating method with direct polymerization on the filler particles as described in U.S. Pat. No. 3,519,593 (Bolger), issued July 7, 1970. See also R. W. Hausslein and G. J. Fallick, Applied Polymer Symposia, No. 11, pages 119-134, 1969. None of these efforts proved commercially successful. In a similar vein, U.S. Pat. No. 3,956,230 (Gaylord), discloses the use of maleic anhydride plus a peroxide free radical initiator as a coupling agent system for polyolefins containing "hydroxyl containing fillers". The resulting composites were not fully characterized in terms of possible coupling or decoupling effects resulting from the use of the selected additives. Some treated filler composites showed improvements in ductility, i.e., elongation at break, and impact but no improvement in, or actual decreases in, the tensile strength. In other cases the reverse occurred and only in infrequent instances, such as Examples 2, 6, 16, 17 and 29, were increases reported for both ductility and strength.
The above techniques were further studied for kaolin-filled polyethylene by D. G. Hawthorne and D. H. Solomon, in "Reinforcement of Polyethylene by Surface-Modified Kaolins, " Journal of Macromolecular Science: Part A, Chemistry, Volume 8 (3), pages 659-671, 1974. The article lists the properties of various polyethylene composites containing 20-40 weight percent kaolin filler containing a variety of surface treatments, including some based on the teachings of U.S. Pat. No. 3,471,439 (Bixler et al.). Attempts were also made to develop more acceptable alternatives to these polymer-encapsulated fillers. The data show no significant increases in composite tensile strength, i.e., "breaking stress", in the cases of treated kaolins as compared to those either untreated or treated with nonreactive, i.e., "saturated", coatings. Also, at the 40 weight percent filler loading, only one sample, having a peroxy initiated pretreatment with a mixture of 2-methyl-5-vinyl-pyridine and diethylene glycol diacrylate, showed a significant increase in elongation at break of from 13 percent to 24 percent.
A recent Japanese patent, Japan Kokai No. 55-110, 138 (Tanaka et al.), which issued Aug. 25, 1980, describes the use of triacryloyl hexahydro-s-triazine in combination with free radical initiators in calcium carbonate filled polypropylene and high-density polyethylene, and in talc filled polypropylene. Small amounts of 1,2 polybutadiene were added in two experiments. In this procedure, the filler was first treated and heated with a mixture of the unsaturated compound and a free radical initiator, such as azo-bis-isobutyronitrile, and then melt compounded with the polymer, preferably with the further addition of more initiator, such as dicumyl peroxide. The injection molded specimens showed generally good increases in impact strength and low to moderate increases in tensile strength. No evaluations were reported without the addition of the free radical initiators.
There are many disadvantages associated with the use of peroxide initiated coupling systems in filled thermoplastics. Perhaps the principal one is that of stability. For example, pretreatment of the filler with the combination of a polymerizable monomer and a free radical initiator can cause pre-polymerization on heating or standing rendering the surface treatment ineffective at the time of compounding the promoter with the polymer. Even when satisfactory conditions for so-called integral blending have been established, i.e., when the filler treatment chemicals can be dispersed directly into the filler and the resin mixture at the time of melt compounding, the resulting compounds often have highly variable rheological properties during processing and highly variable mechanical properties after molding or extrusion. This difficulty in achieving reproducible results no doubt has contributed to the general lack of success of peroxide initiated coupling systems in commercial practice. Furthermore, the decomposition products of organic peroxides are notoriously .[.odiferous.]. .Iadd.odoriferous .Iaddend.and confer a characteristic, undesirable smell to the final products similar to that well known from present peroxide cured polyolefin products. Finally, the use of peroxide additives adds to the cost and processing complexity in the manufacturing of filled polyolefin compounds.
Organic silanes are presently the most widely used coupling agents. These agents are used extensively as surface treatments for fiberglass where they serve a number of diverse functions such as protecting the glass fibers from water-induced stress corrosion, from mechanical damage in handling and processing, by improving the bonding of the fibers to various matrix polymers and by preserving the composite strength upon exposure to water. The state-of-the-art of silane treated mineral fillers in thermosetting and in thermoplastic polymers is outlined in a brochure F-43598A by Union Carbide Corporation published in February, 1979 and entitled, "Silane Coupling Agents in Mineral-Filled Composites." The principal commercial use to date of silanes in mineral-filled, non-crosslinked polyolefins is to maintain good electrical insulating properties after water exposure. The reinforcement promoting effect of most silanes in polyolefins is relatively modest for most mineral fillers. The high cost of silanes also detracts from their use with low-cost commodity mineral fillers.
The use of organic titanates as surface treatments for talc and calcium carbonate in polyolefins has been reviewed by C. D. Han et al., in Polymer Engineering and Science, Vol. 18, No. 11, pages 849-854, 1978. The titanates are reported to act as processing aids and to increase the toughness and the elongation at break of the composites. However, the tensile strength in the best cases is only slightly increased and in most cases it is actually reduced. Hence, referring to most titanates as coupling agents is a misnomer and the disclosed compounds do not belong to the class referred to here as reinforcement promoters.
Other materials such as fatty acids, i.e., stearic acid, fatty acid salts, such as zinc or calcium stearate, various detergents, oils and waxes are commonly employed as compounding ingredients or as pretreatments for mineral fillers in polyolefins. Generally, their effect is similar to that reported above for the titanates in that they facilitate filler dispersion and processing and often increase the elongation at break and sometimes the toughness. However, they do not improve the composite strength and often they even reduce the tensile properties relative to that of the filled polymer without the additive. The state-of-the-art of "hydrophobic" additives has recently been reviewed by D. E. Cope in reprint no. 24-E entitled, "Hydrophobic Filler Wetting-- A New Technique for Improved Composite Performance and Production", from a talk presented at the 1979 Annual Technical Conference of the Reinforced Plastics/Composites Institute of the Society of the Plastics Industry. Another typical example of a recently developed organic filler treatment additive is described by de Souza et al. at pages 492-496 of the preprints from the 1979 Annual Technical Conference of the Society of Plastics Engineers, entitled "Low-Cost Highly Filled Impact Resistant Thermoplastics Composites". The article discloses that the tensile strength of a CaCO.sub.3 -filled polypropylene decreases rapidly with increasing concentration of the filler treatment additive. Hence, these types of filler treatment additives also are not reinforcement promoters as defined herein.
Certain organic compounds are effective coupling agents in specific filler/polyolefin composites. For example, 2,6-dimethylol 4-alkyl phenols dramatically increase the tensile strength of chrysotile asbestos/polyolefin composites up until enough coupling agent is added corresponding to monomolecular coverage of the filler surface. See the article by F. H. Ancker et al., entitled "A Coated Asbestos with Better Coupling", Plastics Engineering, pages 32-36, July 1974. However, these organic compounds are rather specific to the brucite surface of chrysotile and they do not in general provide significant property improvements with particulate mineral fillers.
Certain simple organic chemicals, such as acrylic acid, have some reinforcement promoting effect in isolated filler/polyolefin composites such as aluminum trihydrate and CaCO.sub.3 -filled polyethylene. However, acrylic acid has a high vapor pressure and is quite noxious, both when used as a filler pretreatment or as an integral blend additive during hot melt compounding, and is has therefore not found wide-spread commercial use.
Chloroparaffins are effective coupling agents in mica-filled polypropylene as described in S. Newman and F. J. Meyer in "Mica Composites of Improved Strength", Polymer Composites, Volume 1, pages 37-43, September 1980. While these compounds are true coupling agents by the terms of this invention, they are not reinforcement promoters because they do not simultaneously improve tensile and impact strength. This was clearly recognized by the authors, at page 41 of the above article, who stated: "The impact behavior as measured by Izod impact values for the coupled systems are generally lowered and appear to be dominated by the reduced strain to yield and failure (ductility) of the matrix in the coupled systems. Moreover, this effect is seen to override the increased strength exhibited by these systems."
In summary, the state-of-the-art prior to the disclosure of the present invention is that most so-called coupling agents for filled thermoplastic polymers are not reinforcement promoters by the terms of this invention, i.e., they do not at the same time improve the strength and the ductility (elongation and toughness) of the filled polymer. In instances where some reinforcement promotion has occurred, it has been limited to the use of particular materials or processes which are either costly, inefficient or noxious in use; has been limited to highly select filler-polymer systems; has required the simultaneous use of free-radical generating additives with their associated problems of stability, odor generation, and the like; or has possessed a combination of these detracting features which have severely limited its commercial utility.